1. Field of the Invention
The present invention relates to a process for reducing a concentration of carbon monoxide in a hydrogen-containing gas using a catalyst, and a catalyst used therefor. More particularly, the invention relates to a process for reducing a concentration of carbon monoxide in a hydrogen-containing gas prepared by reforming hydrocarbons, methanol, etc., which is contemplated to be used as a hydrogen source of fuel cells.
2. Prior art
Hydrogen-containing gases have been produced by subjecting hydrocarbons, methanol or the like to steam-reforming, and used as raw materials for various organic chemical products. Recently, particular attention has been paid to the use of these hydrogen-containing gases as a hydrogen source of fuel cells.
However, these hydrogen-containing gases inevitably contain carbon monoxide. When such hydrogen-containing gases are used as a hydrogen source of fuel cells, the carbon monoxide is absorbed onto a platinum catalyst contained in electrodes of the fuel cell, resulting in deteriorated activity of the catalyst. Therefore, it is required to reduce a carbon monoxide concentration in the gas to a level as low as possible. The acceptable concentration of carbon monoxide in the gas is, e.g., several % or lower when used in phosphoric acid-type fuel cells, or several tens ppm or lower when used in solid polymer-type fuel cells.
Methanol can be reformed with steam at a relatively low temperature. The resultant reformed gas has a low content of carbon monoxide and, therefore, can be advantageously used as a hydrogen source of fuel cells. For this reason, many attempts have been made to develop fuel cells using methanol as a raw material, especially those installable in automobiles.
As to methods of reducing a carbon monoxide concentration in the hydrogen-containing gas by oxidizing the carbon monoxide into carbon dioxide using a catalyst, Japanese Patent Application Laid-Open No. 5-245376 discloses the method of converting carbon monoxide into carbon dioxide in the presence of steam using a catalyst composed of copper oxide, aluminum oxide and magnesium oxide. Japanese Patent Application Laid-Open No. 8-295502 discloses the method of selectively oxidizing and removing carbon monoxide using a catalyst composed of metal oxide and gold superfine particles dispersed and supported on the metal oxide. Japanese Patent Application Laid-Open No. 9-30802 discloses the apparatus for reducing a carbon monoxide concentration in a hydrogen-containing gas produced by reforming methanol using a platinum/ruthenium catalyst. Japanese Patent Application Laid-Open No. 11-102719 discloses the apparatus for selectively oxidizing carbon monoxide contained in a hydrogen-rich gas using a catalyst prepared by adding ruthenium to alkali metal.
However, in the above conventional method using the catalyst composed of copper oxide, aluminum oxide and magnesium oxide (Japanese Patent Application Laid-Open No. 5-245376), the reaction temperature is as high as 300 to 400xc2x0 C., and the conversion rate of carbon monoxide is low. In the method using the catalyst composed of metal oxide and gold superfine particles dispersed and supported on the metal oxide (Japanese Patent Application Laid-Open No. 8-295502), although the reaction can be conducted at a low temperature, the catalyst used therein is too expensive. In the method using the platinum/ruthenium catalyst (Japanese Patent Application Laid-Open No. 9-30802) or the method using the catalyst prepared by adding ruthenium to alkali metal (Japanese Patent Application Laid-Open No. 11-102719), these catalysts are also too expensive, and a part of carbon monoxide tends to be reacted with hydrogen to form methane. The formation of methane (methanation) not only consumes a large amount of hydrogen, but also generates a large amount of heat upon the reaction between carbon monoxide and hydrogen, thereby causing significant increase in temperature and making it difficult to control the reaction temperature.
An object of the present invention is to provide a process for effectively reducing a carbon monoxide concentration in a hydrogen-containing gas obtained by reforming methanol or the like for the purpose of developing fuel cells, and a catalyst used therefor.
As a result of extensive researches concerning process for reducing a carbon monoxide concentration in a hydrogen-containing gas, the present inventors have found that a carbon monoxide concentration in the hydrogen-containing gas is effectively reduced by contacting the carbon monoxide with oxygen in the presence of a catalyst comprising platinum and at least one metal selected from the group consisting of cobalt, nickel, copper and manganese. The present invention has been accomplished based on this finding.
Namely, in accordance with the present invention,. there are provided a process for reducing a concentration of carbon monoxide in a hydrogen-containing gas which comprises the step of contacting the carbon monoxide with oxygen in the presence of a catalyst comprising platinum and at least one metal selected from the group consisting of cobalt, nickel, copper and manganese; and a catalyst for reducing a concentration of carbon monoxide, comprising a support and a catalyst component which is supported on the support and comprises platinum and at least one metal selected from the group consisting of cobalt, nickel, copper and manganese.